The present invention relates to a semiconductor device. In recent years, high-density mounting has become a useful elemental technology for achieving compactness, light weight and high performance in cellular telephones, digital video cameras and digital still cameras. In some cases, CSP (chip scale packages) consisting of single chips are inadequate to achieve high-density mounting, and packaging methods such as MCP (multi-chip packages) and SiP (System in Packages) have become more common.
The circuit boards, such as interposers and motherboards, used in composite semiconductor chip packages, such as MCP and SiP, require connections between semiconductor chips on a circuit board, so buildup substrates with high wiring density have come into use. Additionally, flip-chip mounting is becoming standard as a method for mounting semiconductor chips to circuit boards.
Here, flip-chip mounting refers to a method of mounting a semiconductor chip to a circuit board, wherein the semiconductor chip surface and circuit board are electrically connected, not by wire connections such as by wire bonding, but rather by projecting terminals known as “bumps” which are arranged in an array.
Flip-chip mounting is held to have the advantage of enabling the mounting area to be reduced as compared to wire bonding. Flip-chip mounting has the additional characteristic of having short wiring and therefore good electrical properties. Flip-chip mounting is suited to the circuits in portable devices, for which there is a strong demand for compactness and light weight, as well as to high frequency circuits in which the electrical properties are important.
The functions of the circuit boards to which the semiconductor chips are to be connected by flip-chip mounting are varied, including packaging substrates such as interposers and main substrates such as motherboards, as are the shapes and types which include both rigid substrates and flexible substrates. Flip-chip mounting is also used in “chip-on-chip” mounting methods for connecting semiconductor chips with each other. Flip-chip mounting is also known as flip-chip bonding, which is a term indicating a bonding method. Additionally, flip-chip mounting is also sometimes abbreviated as FC mounting or FC processing.
In semiconductor elements that are flip-chip mounted, the gap between the semiconductor element and the circuit board is usually filled with a reinforcing resin composition (underfill) to make the semiconductor element, circuit board and junction portion reliable. Such underfills need to be capable of filling into small gaps in a short period of time, be free of voids and filler segregation, have excellent adhesiveness to various materials, and achieve sufficient reliability for the semiconductor element, the circuit board and the junction portion. As materials for underfills, thermosetting resins typified by epoxy resins have been widely used. For example, JPH11-233571A discloses a semiconductor device using a thermosetting resin having specific properties as an underfill.
According to this publication, the semiconductor device is such that the active surface of a silicon chip is made to face a circuit board and electrically connected to the circuit board via a conductive material, and the gap between the silicon chip and the circuit board is filled and cured with a thermosetting resin composition. This thermosetting resin composition comprises a straight aliphatic hydrocarbon compound having at least 10 and at most 30 carbon atoms, capable of chemically binding to a thermosetting resin. According to the publication, this composition has high temperature cycling reliability while enabling the silicon chip to be removed at a low temperature with little shear, and without damaging the silicon chip or the circuit board.
Additionally, in the field of interposers, the reduced thickness of core materials, the use of coreless structures in which no core is provided, and layered bodies having wiring patterns formed on resins are used as interposers have enabled the development of thinner buildup type interposers having reduced overall interposer thickness and adapted to high frequencies by shortening the interlayer connection length to meet demands for support of higher density mounting and higher operating frequencies.
However, in these thin buildup type interposers, deformation of the interposers can easily occur due to changes in temperature conditions, and differences in the thermal expansion of the interposer, semiconductor chip and underfill can cause the stresses from ambient temperature changes to damage or destroy the solder bumps.
An example of a technology for the purpose of suppressing damage and destruction to solder bumps due to stresses from such ambient temperature changes in thin buildup type interposer with a coreless structure not having a core is the semiconductor device disclosed in JP 2006-24842A.
According to this publication, the semiconductor device comprises a semiconductor element and an interposer with a coefficient of thermal expansion of at least 16 ppm/° C. on which the semiconductor element is mounted, connected by solder bumps, wherein the gap between the semiconductor element and the interposer and the gaps between the solder bumps are filled with an underfill resin which is then cured. Additionally, the filler resin has a glass transition point at a temperature of 100-120° C., a modulus of elasticity at 125° C. of at least 0.1 GPa and a coefficient of thermal expansion a1 of at most 30 ppm/° C. at lower than the glass transition point. Furthermore, this publication describes that with this composition, it is possible to reduce damage and destruction between the semiconductor element, the interposer and the printed circuit board.
However, the conventional art described in the above publications have room for improvement in connection with the following points.
First, circuit boards and semiconductor chips usually have different coefficients of thermal expansion. Therefore, when a semiconductor device having a semiconductor chip mounted on a circuit board is subjected to a heat history, the difference in coefficient of thermal expansion can cause the circuit board to warp.
While the semiconductor device described in JP H11-233571A pertains to a thin buildup type interposer using a coreless structure that does not have a core, it does not consider thin buildup type interposers having a structure with a reduced core thickness. Generally, the coefficient of thermal expansion completely differs between coreless structures and structures having cores, so the degree of deformation of the interposer due to changes in temperature conditions will completely differ.
Secondly, in recent years, materials different from those that are conventional have come into use as the materials for forming semiconductor chips and materials for the bumps connecting the semiconductor chips to circuit boards, and underfills have also been newly designed in connection therewith. For example, with regard to semiconductor chips, low dielectric-constant films known as “low-k” have come into use as insulating film materials for forming wiring layers. By using such films, it is possible to suppress cross-talk between wires to result in semiconductor devices that operate at high speeds with high reliability. Additionally, materials not containing lead are becoming standard as the materials for bumps in consideration of friendliness to the environment.
The occurrence of damage such as cracks indicated above becomes more marked when low-k layers or lead-free solder are used. Low-k films generally do not have adequate mechanical strength. Therefore, when stresses occur in a semiconductor chip due to the package warpage or the like, cracks may appear in the low-k film even if the degree of stress is not very high. Additionally, since lead-free solders do not have adequate toughness, they can tend to form cracks in the interfaces between the bumps, the semiconductor chip and the circuit board.